Many types of electronic apparatus are known in which the various electrical components are interconnected by conductors. The interconnecting conductors are fabricated in a wide variety of processes such as, for example, thick film fired conductor systems, polymer conductors and printed circuit boards.
In thick film fired conductors, a mixture of a conducting metal powder, a ceramic or glass binder and an appropriate vehicle is screen printed on a substrate. The conductor pattern on the substrate is then fired at a relatively high temperature, typically between 650.degree. and 900.degree. C. As the temperature rises to the firing temperature, the vehicle is volatilized leaving the metal and binder behind. At the firing temperature, sintering of the metal takes place to a greater or lesser extent with the binder providing adhesion between the metal film formed and the substrate.
Thick film fired conductors have classically employed precious metals such as gold, silver, platinum and palladium. Recently these noble metals have soared in cost, and new conductor systems using copper, nickel and aluminum are being made commercially available. The cost of the precious metal systems is prohibitive where a low cost conductor system is desired. The newer metal systems are not significantly cheaper because of the special chemistry which is required to prevent oxidation of the metal during the firing process. Moreover, these systems are very difficult to solder using the conventional tin/lead solder and the high firing temperatures required during fabrication preclude the use of low cost substrate materials. Some of the nickel systems can be fired on soda-lime glass at temperatures just below the melting point of the glass but the resulting conductivity of the conductor is relatively low.
The term "polymer conductor" is actually a misnomer since the polymer is not actually a conductor. Instead, the polymer is heavily loaded with a conducting metal and screened on to a substrate. The advantage of this system is that the polymer can be cured either catalytically or thermally at temperatures which range from room temperature to about 125.degree. C. As a result of this so-called "cold processing", it is possible to use very inexpensive substrates such as films of Mylar (polyethylene terephthalate). The mechanism by which conductivity is achieved is supplied entirely by contact between individual metallic particles. It has been found that the only metals which can be loaded into the polymer and give acceptable conductivity are the precious metals such as gold and silver. All of the other standard conducting metals oxidize over a period of time reducing the conductivity between the particles. Silver has been the predominant choice in polymer conductor systems but the silver systems are generally not solderable because the silver is leached by the lead-tin solder. When the price of silver is about $10-11 per ounce, these conductor systems are competitive with other systems if used on very low cost substrates such as thin mylar films. However, when the price of silver is higher, the systems are not competitive with printed circuit boards.
The techniques used to prepare printed circuit boards can be divided into additive and subtractive technologies. In both, the starting point is a substrate, which can vary widely from phenolics to glass filled epoxies, on which a copper foil is bonded. In the additive preparatory system, the copper foil is very thin, usually on the order of about 200 microinches. A resist is patterned such that the copper is exposed only where the conductors are desired and the board is then electroplated to form copper conductors of about 1 mil in thickness. The plating resist is stripped and the copper is etched. In areas where the conductor is not desired, the copper is only about 200 microinches thick so that etching quickly removes this copper while leaving a 1 mil thick conductor. In the subtractive process, the starting thickness of the copper foil is usually between 1 and 2 mils. An etch resist is deposited wherever the conductors are desired, the board is etched and the resist is then removed. The resist prevents etching where the conductors are desired leaving conductor runs.
Both the additive and subtractive printed circuit board procedures require the application of a copper foil over the entire substrate, deposition and removal of a resist, etching of the printed circuit board, drilling holes for component insertion, and in one case, the additional step of electroplating. An advantage of this technology is, however, that the resulting circuit boards can be relatively easily soldered.
The most significant drawbacks of the printed circuit board technology is that a substantial number of processing steps are necessary and this requires a large amount of associated equipment. In addition, the choice of substrate materials is limited to one of those available for circuit board materials. The number of process steps and equipment results in relatively high processing costs and the limitation of the substrate materials eliminates the opportunity to use a decorative or structural member which may be required in the apparatus as the substrate. Typical total costs for processed printed circuit boards range from $0.03 to $1.00 per square inch depending on the quality of the board, whether single sided or double sided and whether plated through holes are used.
Providing reliable electrical connections to some substrates have posed particular difficulties. One system which has posed a continual problem is liquid crystal cells where the connection must be made to a multiplicity of tin oxide conductor patterns which are formed on a glass surface. This connection has usually been made by mechanical means, such as by pressure contacts, but such pressure contacts to tin oxide patterns on glass are fraught with problems. The contact must be extremely compliant because the glass is essentially non-compliant and this becomes a greater factor as the number of connections required increases. The problem is complicated by the fact that most materials do not form good ohmic contact with tin oxide patterns on glass and, additionally, there is no way to index the contact because there is effectively no height associated with the tin oxide pattern rising above the glass surface. Still further, if the glass is scratched, the tin oxide conductor is also scratched and an open circuit results. Finally, it is desirable in many installations to mount the liquid crystal cell flush with the surface and in such installations, there is no room for a complex connector.
The prior art techniques are not particularly adapted to forming electrical conductor interconnects with such hard to connect substrates. For example, the conventional thick film techniques generate several additional problems because the firing step required raises the temperature to close to the melting point of the glass and, therefore, tends to warp the glass which is extremely undesirable in liquid crystal technology. The firing also tends to form an oxide film on the tin oxide resulting in a rainbow effect which is also undesirable in a display cell.